Dimethyl methyl phosphonate, a flame retardant and candidate nerve gas simulant, was tested for reproductive toxicity in male Fischer 344 rats and B6C3F1 mice. Male rats and mice were dosed for 13 weeks at doses of 0, 500, 1000 and 2000 mg/kg and then mated to untreated females. Female rats and mice were sacrificed 14 days (rats) and 19 days (mice) after mating and pups examined. There was a dose related decrease in litter size and an increase in number of resorptions. Histologic abnormalities were seen in the testes of the high dose male rats. Histologic evaluation of the mouse tissues is still in progress.